1. Field of the Invention
This invention relates to spatial light modulators, more particularly to reflective spatial light modulators used in imaging applications.
2. Background of the Invention
Reflective spatial light modulators typically comprise arrays of individually controllable elements. The array receives light from a source and each individual element is controlled to either reflect light to an imaging surface or not.
In most applications the ON state for each element reflects light to the imaging surface. In some applications, the OFF state reflects light to the imaging surface. One problem common to either arrangement occurs in separation of the two light paths. Light from elements in the OFF state must be widely separated from light in the ON state, or the ratio between the ON and OFF states (the contrast ratio) will be too low.
Contrast ratio is the ratio of the image when it is all dark or black and when it is all white. Stray light entering the imaging system and becoming projected to the imaging surface in the all black state raises the black level relative to the white, lowering the contrast ratio.
One solution to this involves reflecting light from OFF elements in the opposite direction from the ON elements. However, some light from the modulator may still stray into the ON path, even when the entire picture is supposed to be OFF, or black. This light normally reflects off the modulator structure itself. For example, micromechanical modulators typically achieve the ON and OFF states by physically moving from one state to another. That movement may permit light to pass between mirror elements. After several reflections within the modulator superstructure it reemerges between mirror elements and enters the projection optics. Though all of the elements of the modulator were OFF, this `stray light` is detected and reduces the white/black ratio. Reflective modulators include micromechanical modulators that cause the elements to physically move from the ON state to the OFF state. Others include liquid crystal devices that cause the crystalline material to either absorb or reflect light In either example, among others, light from the modulator's structure surrounding the elements will enter the ON path, regardless of the state of the individual elements. This has an adverse effect on the black levels, degrading the contrast ratio.
Therefore, a solution is needed that eliminates the stray light from the ON state, while not adversely affecting the manufacturing costs or complexity. The manufacturing process of the modulator should include the solution without too much increase in complexity or time, keeping costs down.